The invention relates to a method for manufacturing products made of plastics.
Specifically, but not exclusively, the method can be used for making caps for containers provided with a tamperproof band having a flap portion folded inwards, this flap portion being in particular configured for forming an abutting element that cooperates with the neck of the container in order to detach the tamperproof band when the container is first opened.
A method is known for folding an annular flange of a tamperproof band of a cap made of plastics according to which the cap, when it has just been extracted from the mould, has the flange orientated with a tilt that is different from the tilt thereof when in use; the cap is then cooled to ambient temperature, after which the flange is folded mechanically to the use configuration.
From EP 1165391 a method is known for making a cap made of plastics provided with a flexible band that is moulded in the use configuration (folded towards the inside of the cap), and in which the spring-back of the band to the original forming position thereof is forced mechanically immediately after moulding of the cap, by an element of the forming mould or by another element that acts after extraction of the cap from the mould, before cooling of the band.
From U.S. Pat. No. 5,846,471 a method is known for moulding a closure with a safety band provided with flaps extending radially inward and upwards, in which the flaps are first moulded with the desired tilt, then the mould is opened by temporarily altering the angle of tilt of the flaps, lastly the flaps return to the original tilt through the effect of the contact with a part of the mould.
Various aspects of the prior art methods are improvable. First, it would be desirable to be able to decrease the effect of the spring-back of the material after the element made of plastics has been folded, so as to reach easily and reliably a final configuration that is as close as possible to that desired, this drawback being detectable in particular when folding is performed hot. Second, it would be desirable to minimise the yield effect of the material near the folding zone, so that the connection between the folded element and the rest of the workpiece is not weakened, this drawback being detectable particularly when folding is performed cold.
Specifically, but not exclusively, the method can further be used to make caps for containers provided with a skirt portion and with a tamperproof band intended for being separated from the skirt portion when the container is first opened (in order to provide clear proof of opening) through the effect of the breakage of a facilitated breaking zone. The tamperproof band is generally provided with a contrasting arrangement that may comprise, by way of non-limiting example, an inwardly folded flap portion. In particular this contrasting arrangement can be configured to form one or more abutting elements that cooperate with at least one protrusion on the neck of the container in order to detach the tamperproof band when the container is first opened.
Forming the aforesaid facilitated breaking zone by using cutting devices that operate so as to perform a cut (continuous or discrete) along a circumferential zone of the skirt portion is known. Some examples of cutting devices of known type used for this purpose are shown in the patent documents EP 0619168, WO 99/17911, WO 00/44538, WO 2004/004993.
One of the problems of known forming systems of the facilitated detachment zone of a cap made of plastics with tamperproof band consists of the fact of ensuring a clean and precise cut in the plastic material, such as to avoid the risk of anticipated and undesired detachment of the tamperproof band, and simultaneously to ensure that when the container is first opened the band is detached correctly with facility and cleanly. It is therefore necessary or at least advantageous to make the cut within precise tolerances. Another problem is that of performing the cut rapidly in order to increase productivity.
A further problem is that of choosing suitable conditions for cutting in function of the type of product to be cut, in particular according to the type of plastic material and of the form of product.
Another problem arises if the cutting of the material for forming the tamperproof band coincides with (generally immediately before or after) the guided mechanical folding of the possible flap portion, this folding generally being conducted after moulding to bring this flap portion (when present) to the use configuration in which it faces inwardly the cap and upwards (with reference to the use configuration in which the cap closes the upper mouth of the container that is arranged standing). This folding is generally necessary because in the use configuration the flap portion forms an undercut that would not allow removal from the mould. If the folding step is conducted near the place or time with the step of cutting the tamperproof band, there is thus the problem of finding a work condition that is suitable for correctly performing both operations.